Conventionally, the nearest neighbor algorithm, bilinear algorithm (linear interpolation method), bicubic algorithm (cubic interpolation method), pixel-averaging algorithm, and the like have been used to obtain an output image of a size different from that of the input image.
The pixel-averaging algorithm is a method of reducing the size of a source image by averaging pixel values in the source with consideration for the area that the reduced image (output image) occupies in the source image. In most cases, reduced images obtained with the pixel-averaging algorithm are of higher quality than reduced images obtained using other algorithms.
One technique using the pixel-averaging algorithm disclosed in Japanese Patent Application Publication No. 2003-256827 finds average values of pixels after first enlarging the size of the source image to the lowest common multiple of the size of the reduced image.